Collated screws are known in which the screws are connected to each other by a retaining strip of plastic material. Such strips are taught, for example, by U.S. Pat. No. 4,167,229 issued Sep. 11, 1979 and related Canadian Patents 1,040,600 and 1,054,982, the disclosures of which are incorporated herein by reference. Screws carried by such strips are adapted to be successively incrementally advanced to a position in alignment with and to be engaged by a bit of a reciprocating, rotating power screwdriver and screwed into a workpiece. In the course of the bit engaging the screw and driving it into a workpiece, the screw becomes detached from the plastic strip leaving the strip as a continuous length.
In the use of such collated strips in screwdrivers, the strip serves a function of assisting in guiding the screw into a workpiece and, to accomplish this, the strip is retained against movement towards the workpiece. In the strip, each screw to be driven has its threaded shaft threadably engaged in a threaded sleeve of the strip such that on the screwdriver engaging and rotating each successive screw, the screw turns within the sleeve which acts to guide the screw as it moves forwardly into threaded engagement into the workpiece. Preferably, only after the tip of the screw becomes engaged in the workpiece, does the head of the screw come into contact with the sleeves. Further forward movement of the screw into the workpiece then draws the head downwardly to engage the sleeve and to rupture the sleeve by reason of the forward movement of the head with the stirp retained against movement towards the workpiece. The sleeve preferably is configured to have fragible straps which break on the head passing through the sleeve such that the strip remains intact as a continuous length. Since the strip is a continuous length, on advancing the strip with each successive screw to be driven, it necessarily results that portions of the strip from which each screw has been driven are also advanced to exit from the power screwdriver.
Known power screwdrivers for driving such collated strips include U.S. Pat. No. 4,146,071 to Mueller et al, issued Mar. 27, 1976, and U.S. Pat. No. 5,186,085 to Monacelli, issued Feb. 16, 1993, the disclosures of which are incorporated herein by reference. Such known power screwdrivers include a rotatable and reciprocally moving screwdriver shaft which is turned in rotation by an electric motor. A screwdriving bit forms a forwardmost portion of the shaft for engaging the head of each successive screw as each screw is moved into a driving position, axially aligned under the screwdriver shaft.
An important aspect of such power screwdriver is the manner and accuracy with which each successively advanced screw is positioned so as to be properly aligned axially under the screwdriver shaft for successful initial and continued engagement between the bit and the screwdriver head in driving a screw fully down into a workpiece. In the devices of Mueller et al and Monacelli, the strip is effectively held in position, notably, by reason of the device grabbing and fixing the position of the screw beside the screw to be driven. These devices also teach location of a screw to be driven in part by the head of the screw to be driven engaging the surface in a guide channel in which the screwdriver bit is reciprocal. In each of Mueller et al and Monacelli, a specific footplate is provided to grasp and fix the position of the screw beside the screw to be driven. The footplate engages a workpiece and is spring biased towards the workpiece. On the device being brought into engagement with the workpiece, the footplate retracts towards the device. The footplate has a conical recess which engages the tip of the screw next to the screw to be driven and applies a force to that screw pushing it rearwardly so that its head bears on a locating plate in the screw feed mechanism. By reason of this next screw being firmly clamped between the footplate and locating plate, the strip is effectively locked into position and thereby positions the screw which is to be driven. Such footplates, however, have the disadvantage of being separate movable parts which must move forwardly to permit successive screws to be advanced and then rearwardly to clamp the next screw.
Another disadvantage of prior art devices in which the device grabs the screw beside the screw to be driven is that such systems do not permit the very last screw in any strip to be driven without possible difficulties. The last screw therefore frequently has to be discarded and/or may result in jamming. This is a particular disadvantage where screws are collated into strips which do not have a large number of screws. For example, in a simple strip of twenty-four screws, to discard every twenty-fourth screw is a substantial disadvantage.
Another disadvantage of prior art devices appreciated by the applicant is that screw advance mechanisms such as those used in U.S. Pat. No. 4,146,071 utilize pawls which on withdrawal of the pawl so as to be in a position to advance the next successive screw of the screwstrip tend to frictionally engage and, to some extent, withdraw the entire screwstrip when this is not required. Such "feed pawl drawback" is disadvantageous insofar as it can withdraw a screw to be driven from axial alignment with the driver shaft.